Automotive LED Driver Power Conversion Topology Guide

This article aims to simplify the selection process by explaining the benefits, trade-offs, and applications for different switching topologies used for LED drivers. Utilizing specific semiconductor junctions, LED manufacturers can produce specific colors of light spanning the entire visible range as well as IR and UV.

Output from the LED is well-controlled light that relies upon accurately regulated current the junction irrespective of small fluctuations at the terminal voltages.

Switching topologies for Automotive LED Systems

Implementing a particular switching topology is chosen in accordance with the complete system design where following factors must be considered:

• Minimum input voltage
• Maximum input current
• Maximum string voltage
• Chassis return capability
• Shorted output capability
• Output LED current
• PWM dimming

Step-Down (Buck) Converters

A buck converter is a type of DC-DC step-down regulator that efficiently converts a higher DC input voltage to a lower adjustable output voltage. Step-down (or buck) LED drivers regulate the current in an LED string from a voltage that is higher than the total LED string voltage. Buck converters are employed when the load voltage (i.e., the voltage required by the LED) is never more than about 85% of the supply voltage. Buck LED drivers have the capability of chassis return and they can be safely shorted to the system ground.

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Figure 1: Buck Converter (Source: Analog Devices)

Following are the several critical features that should be looked for while implementing the buck converters:

  • Fixed frequency operation
  • High efficiency through excellent switching control and low resistance switches
  • High accuracy throughout the analog dimming range
  • For excellent EMI, a properly designed spread spectrum frequency modulation

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Figure 2: Buck Converter Example LT3932

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Step-Up Converters

A boost converter (or step-up converter) is a type of DC-to-DC converter that increases voltage while decreasing current from its input to its output. It efficiently steps up the input voltage to a higher output voltage by storing energy in an inductor during the switch-on phase and releasing it to the load during the switch-off phase.

Step-up (or boost) LED drivers regulate the current in an LED string from a voltage that is lower than the total LED string voltage. This is useful in many automotive systems, where many LEDs need to conduct in a single string. Typical 12 V automotive systems have operational ranges from 6 V to 18 V—requiring that the LED driver runs down to 6 V, providing large step-up ratios for the 187100735 LEDs to remain illuminated.

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Figure 3: Boost Converter (Source: Analog Devices)


Figure 4: Boost converter example LT8356-1

Boost-Buck Using a Boost Converter

Some step-up (or boost) LED drivers may be configured to return the LED cathode to the supply. This configuration is referred to as buck-boost. The total output voltage is VIN (VBATTERY), which is added to the total LED string voltage.

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Figure 5: Boost-buck converter (Source: Analog Devices)

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Figure 6: Boost-buck converter: LT8386

Buck Mode Using a Boost Converter

Some step-up (or boost) LED drivers may be configured to step-down from the supply (rather than ground referenced, as in a standard buck)—creating a buck-mode configuration. This configuration has the same limitations as a buck, where the total LED string voltage must be less than the input supply.

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Figure 7: Buck-mode converter (Source: Analog Devices)

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Figure 8: Buck-mode example: LT3756-2

Buck-Boost Converter
Buck-boost LED drivers regulate LED current from a supply that is higher or lower than the total LED string voltage. The converter modulates the high-side switch connected to the input voltage in the step-down mode and the low-side on the output-side in step-up mode. This topology is the most complex but also the most flexible. VIN and VOUT ranges are only limited by the controller IC. This is a good choice for matrix applications.

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Figure 9: Buck-boost converter (Source: Analog Devices)

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Conclusion

Automotive LED lighting systems can be driven with switching regulators in many different ways. Depending on the application, the selection of switching topology and configuration allows the lighting designer to create complete subsystems for the different lighting requirements throughout an automobile. Selecting the correct power conversion switching topology and configuration for the system optimizes requirements such as complexity, efficiency, EMI, and safety.